This study will evaluate the acquisition of immunologic function in infants and children with DiGeorge Syndrome. In this syndrome, children are born with insufficient thymic tissue and T cell development is delayed and impaired. Secondary B cell defects may occur as well. This study will document the dynamic nature of the immunologic development in this patient population. Laboratory evaluation of T cell production and T cell function at four time points in infancy will define the acquisition of T cell function. A cohort of older children will have single time point evaluations of T cell production, T cell function, and B cell function. Sixty children with DiGeorge Syndrome and 10 controls have been evaluated to date. During the next year, we will enroll approximately 12 more patients and fifteen controls. This study will have important implications for the management and care of this patient population.